Over the years, with the advent of electronics and later computers, hundreds of electrical devices have been developed. With the ability to assemble sophisticated and complex electrical devices has come emphasis on reduction in size, ease of assembly, reduction in number of component parts and total cost reduction. Because of the development of microprocessors and microcomputers, it has become possible to assemble smaller, and more compact electrical devices. Although these devices use different families of parts, circuits and components, assembly of these electrical devices is effected by the interconnection of the electrical circuitry, generally mounted onto flat, substantially two-dimensional circuit boards using printed wiring assemblies with attached components to carry, control, select, store and manipulate the electronics signals as required for each device. Although manual insertion and soldering is a popular approach for joining electrical and electronic components to circuit boards, it is a labor intensive procedure with an inherent propensity for non-uniformities, and at times, inaccuracies.
At the location of the interconnections of the components, circuit boards and wiring assemblies, the greatest potential for system failure exists. Because of the size, tolerance requirements and relative position associated with the attachment of the electrical components to conventional circuit boards, usually printed circuit boards (PCBs), and electrical circuit interconnection of the PCBs, much of this assembly is performed by hand. For attachment of an electrical component to a PCB, components leads are inserted through holes that are tooled into the PCB and the component leads are then usually either wave soldered or hand soldered to the PCB, rigidly attaching the component into position.
Prior to this invention, this procedure remained substantially the same for interconnection of the PCB electrical circuits, for example, if a ribbon connector or an edge connector was used, it had to be attached to each PCB in the same manner used for attachment of a component. PCB interconnection usually involved hand soldering of connectors, ribbons or pins, which was especially true for the display and mother boards.
Many practical difficulties are associated with the assembly of these small electrical devices. Assembly required some degree of expensive, non-uniform hand work; the hand soldering techniques typically were performed at individual assembler stations where the soldering procedure was repeated numerous times depending on the number of interconnection points required. The soldering points present sites for system failure due to insufficient or improper attachment; reduction in the number of soldering points reduces failure locale. Moreover, due to the cost of labor, the soldering operations and their number substantially affected the cost of the units produced. In addition, although surface mount technology was available, it could not be utilized to its fullest potential because connector attachment typically required soldering of the leads.
Conventional connectors are not conducive to assembly by automation because:
a. Connector to PCB assembly often requires dedicated equipment; PA1 b. Soldering may have to be manual because of the relative position of the connectors on the PCBs; PA1 c. Through hole connectors may reduce the ability to exploit surface mount component assembly; PA1 d. The mechanical assembly of PCBs with rigid pins and sockets is not easily automated; PA1 e. Externally applied clips and/or brackets are necessary to hold PCBs together; and, PA1 f. All PCBs must be supported to avoid mechanical stress of the electrical connection system. PA1 a. Dual function of parts, thereby reducing part requirements and their associated assembly costs (including labor and fixtures); PA1 b. Reduction in size requirements; PA1 c. Reduction in inspection operations; PA1 d. Increasing reliability and repeatability; and, PA1 e. Minimized field failure opportunities.
For the assembly of small electrical devices, problem areas include: the number of interconnection and the type of interconnection; and, the number of PCBs and components, and their size and space requirements. Depending on the intended function of the electrical device, whether it includes computer software, and its application, the electrical components, attached at different locations on the circuit boards, are of different sizes, and may be numerous. Reduction in PCB, component and connector types, sizes and numbers, is beneficial. For example, although surface mounting of components is available using pick and place machines, because of the through-board requirements of many of the connectors, this type of automation has not been fully exploited.
In addition to electrical interconnection of the circuits, in the prior art, the PCBs had to be structurally mounted to prevent displacement and breakage which would cause error and failure in the electrical device. This assembly step also was labor intensive and did not lend itself to automation.
Moreover, for small electrical devices with industrial process applications, usually operator interface is necessary. For these devices, the desired operator interface with the process system is achieved by using a combined mother/display board and an operator input device such as a switch. This type of interface, using a combined mother/display board has specific soldering problems in that the solder joints need to be made on both sides of the PCB because of the interface requirements. However, conventional wave soldering techniques are only compatible with through-hole component attachment on one side of the PCB. Therefore, attachment of components on the second side of the PCB is by hand soldering. To solve this problem, flexi-rigid PCBs are used. The flexi-rigid PCB is a PCB of varying thicknesses which combines mother/display board needs by having, for example, two thick portions and one thin portion in one PCB such that the thick portions can be folded over at the thin part of the PCB, effectively creating components on both sides. (Mother boards are circuit boards that interconnect PCBs, e.g., other PCBs may be plugged into a mother board.) However, flexi-rigid PCBs are expensive to manufacture because of their varying thicknesses, and do not provide for interconnection of mixed technology PCBs.
For the small electrical devices used for process applications, the switch was often a membrane switch. These dual layer switches, which are fixed key position switches, typically opaque, must be connected to the mother/display board (which is also referred to herein as a display board or display PCB). Interconnection normally was effected by an flexible integral tail which was a connector ribbon, from one or both of the two layers of the membrane switch, having the conductive traces from the switch extended thereon. The tail or connector ribbon was then connected to the mother/display board by metal pins which were crimped, inserted or soldered, or by plugging the tail into a socket mounted onto the mother/display board. However, the tail was a flimsy structure and its position difficult to predict, so this assembly step was not easily automated.
Although there were some piecemeal attempts to address some of the problems associated with the assembly of small electrical devices, as discussed, many problem areas still existed.